As a starting point, you can check Mac2Sell, eBay (including completed sales), or Amazon for prices of similar models, to get an idea of what a reasonable price may be. Then adjust the price to account for condition, configuration, remaining AppleCare coverage (if any), etc.

$750-900 tops, depending in what do you give to sweeten the deal, good care of the machine, and no cosmetic/functional issues in the equipment.

Take in account shipping costs/fees if selling through an online site. And be aware that the second-hand market is more price-wise, than hardware-wise, with too many buyers searching the lower $ (price) than hardware improvements/add ons.